There was UUCP email, where you had to know the path of hostnames to route the mail. No FQDNs! Just a list of hostnames separated by "!" Example from [0]: "Us

There was UUCP email, where you had to know the path of hostnames to route the mail. No FQDNs! Just a list of hostnames separated by "!" Example from [0]: "User barbox!user would generally publish their UUCP email address in a form such as …!bigsite!foovax!barbox!user." You had to learn how machines were connected, and which were high capacity. Otherwise, delivery might take days, or fail. [0] https://en.m.wikipedia.org/wiki/UUCP#Mail_routing https://en.m.wikipedia.org/wiki/UUCP#Mail_routing